Mechanical Behaviour of Carbon Nanotubes

Mechanical Behaviour of Carbon Nanotubes

Danilo Vuono (Federiciana Università Popolare, Italy)
DOI: 10.4018/978-1-7998-1530-3.ch002

Abstract

Mechanical properties of carbon nanotubes (CNTs) are very interesting for the nanocomposite field. The possibility to use these nanomaterials as fibers in polymeric matrix is one of the most important applications of the last years. This study recognised the mechanical properties of CNTs and the obtained composites with polymeric matrix. The second part of chapter presents a short characterisation of an epoxy-CNTs-based composite. This study verified the hypothesis made concerning a different tensile strength of these materials as a function of presence of structure defects.
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Introduction

Carbon nanotubes are molecular carbon fibers, formed by little graphitic cylinders closed to the extremity by caps of fullerenes. Carbon nanotubes are classified in two families: multi-walled nanotubes (MWCNTs), discovered in 1993 (Ijima & Ichihashi, 1993) and single-walled nanotubes (SWCNTs), observed for the first time by Bethune et al. (Bethune et al. 1993). SWCNTs can be imagined as a graphitic plane rolled upon itself, formed by hexagonal sweaters of carbon atoms with sp2 hybridisation (Iijima, 1991). CNTs have several structural configurations: armchair, zig zag and chiral. Different configurations have different physico-chemical properties, especially for electric conductivity. MWCNTs are formed by several concentric graphitic sheets. CNTs are very flexible and hard materials. The secret of these properties is in the σ carbon-carbon bonds that are present in the graphite (Iijima, 1991; Chiang et al. 2001). Hence, their applications as fibers in nanocomposite materials is developed in the last years (Chiang et al. 2001). Moreover, they are considered as very good thermal conductors with high performances comparable to those of graphite (Popov, 2004). They are also applied in the field of electrical conductors. The SWCNTs can assume a metallic or semiconductor behaviour as a function of the rolling up of the graphitic plane to create the tube (Ajayan & Ebbesen, 1997). In so far, the zig-zag CNTs could always considered as semiconductors and sometimes as metallic conductors in particular structural conditions, while the armchair CNTs are always considered as metallic conductors (Frank et al. 1998). CNTs have presented high sensibility to the electric fields, folding up itself up to 90° and then taking back their initial shape without damages (Baughman & Zakhidov & de Heer, 2002). Carbon nanotubes have a high chemical inactivity. They withstand to the oxidative agents and acids. They present a strong capillary property. Therefore SWCNTs, for instance, are used as ideal adsorbent materials for liquids and gases (Pederson & Broughton, 1992). Adsorption properties of CNTs have been studied for the H2 adsorption in order to optimise new storage techniques (Piérard et al. 2002). Adsorption is a superficial molecular attraction phenomenon between the solid phase, the adsorbent, and a gas or liquid phase, the adsorbate. Adsorption process in a solvent-solute-solid system is strongly affected both by solvent-solute affinity and solute-adsorbent interactions. This is realised through three types of forces: electrostatic forces, van der Waals forces and chemical forces. Li et al. studied Pb, Cd and 1,2-dichlorobenzene adsorption using carbon nanotubes. Indeed, results demonstrate that nanotubes have remarkable adsorption capacities (Li et al. 2007). In particular, the CNTs with structural defects have high adsorption performances, compared to those without structural defects. The interest for this kind of application has allowed to develop several computer simulations using analytical theoretical models (Furmaniak et al. 2006). The interaction between the CNTs and the organic compounds are strongly dependent on nanotubes diameters and, of course, the adsorbent-adsorbate interactions. Liao et al. studied the adsorption properties of the CNTs for phenolic derivates (Liao & Sun & Gao, 2007). Yang et al. detected the adsorption capacities of MWCNTs for the APH (Aromatic Polycyclic Hydrocarbons) (Yang & Zhu & Xing, 2006; Yang & Xing 2006; Yang et al. 2006). In 2009, a Russian research group tested the adsorption interactions between multi-walled carbon nanotubes and benzoic acid in aqueous solutions (Kotel et al. 2009). They determined the optimal functionalised CNTs to be used in this application. Nevertheless, applications using their mechanical properties remain the main field of development for these kinds of materials. Since 1995, Ruoff and Lorents (1995) discussed aspects of the mechanical and thermal properties of carbon nanotubes. The tensile and bending stiffness constants of ideal multi-walled and single-walled carbon nanotubes are derived in terms of the known elastic properties of graphite. Tensile strengths are estimated by scaling the 20 GPa tensile strength of Bacon’s graphite whiskers. It is widely perceived that carbon nanotubes will allow construction of composites with extraordinary strength: weight ratios, due to the inherent strength of the nanotubes. Several “rules of thumb” have been developed in the study of fiber/matrix composites. Close inspection of these shows that carbon nanotubes satisfy several criteria, but that others remain untested (Ruoff & Lorents, 1995). Campbel et al., in 1999, report here the fabrication and characterization of electrodes constructed from single carbon nanotubes. The sigmoidal voltammetric response of these nanotubular electrodes is characteristic of steady-state radial diffusion. They demonstrated that electrochemical nanotubular electrodes can be constructed from single carbon nanotubes. Insulated electrodes of arbitrary length with 80-200 nm diameters can be routinely fabricated. These electrodes represent a new application of carbon nanotubes that takes advantage of their geometrical shape, mechanical strength, and electrical conductivity (Campbell & Sun & Crooks, 1999). Finally, Salvetat et al. present variety of outstanding experimental results on the elucidation of the elastic properties of carbon nanotubes are fast appearing. These are based mainly on the techniques of high-resolution transmission electron microscopy (HRTEM) and atomic force microscopy (AFM) to determine the Young’s moduli of single-wall nanotube bundles and multi-walled nanotubes, prepared by a number of methods. Techniques are used for these objectives for the first time. Collected data show that the Young’s modulus of CNTs is at least as high as graphite and can be even higher for small SWNTs. Experiments show that Young’s moduli for MWNTs are dependent upon the degree of order within the tube walls.

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